It is traditionally maintained that according to Aristotle, matter provides a principle of individuation. Objections of several sorts have been raised against this interpretation. One objection holds that for Aristotle it is form, rather than matter, that individuates. A more radical objection is that Aristotle does not propose any principle of individuation at all. Any adequate discussion of this issue must make clear precisely what problems such a principle is meant to address. This in turn requires that several important distinctions be observed: (1) principles of individuation vs. principles of unity; (2) metaphysical vs. epistemological principles; (3) synchronic vs. diachronic unity and individuation; numerical vs. qualitative identity. After examining the objections in the light of these distinctions, I conclude that the objections cannot be sustained, and that, with appropriate qualifications, the traditional interpretation gives us the right idea about Aristotle.